1. Scope of the Invention
The present invention relates to an apparatus which can simulate any respiratory function in mammals for the purpose of calibrating analytical systems. The simulation is accomplished by a piston pump which "inhales" surrounding atmospheric air, composed approximately of 21% (by volume) oxygen and 79% nitrogen, and mixing said atmospheric air in varying proportions, with a pre-mixed volume of carbon dioxide and nitrogen, then "exhaling" the pre-determined gas mixture to simulate a particular respiratory performance evaluated by the test system to be calibrated.
Respiratory function can theoretically range from zero oxygen conversion (all oxygen inhaled is exhaled) to nearly total oxygen conversion (no free oxygen is exhaled). An approximation of total oxygen conversion can be found in nature in diving whales that can hold their breaths for as long as 45 minutes, during which time most of the oxygen is consumed. The exhaled gases of these diving whales approximate a mixture of about 17% carbon dioxide, by volume, in nitrogen.
The present invention utilizes a mixture of gases which in composition approximates a gas that a diving whale would exhale after total oxygen depletion. By diluting the gas mixture with a pre-determined amount of atmospheric air, any range of respiratory function can be simulated in a known mixture of "exhaled" O.sub.2 --CO.sub.2 --N.sub.2 gas. By comparing the known parameters of the respiratory simulator with the results given by the analytical respiratory equipment being calibrated, the accuracy of such analytical equipment can be verified.
2. Prior Art
Prior to the use of respiratory simulators, in order to calibrate an analytical respiratory equipment system, a biological subject would have its expired gases collected at the outlet of said system and subjected to tedious classical analyses and manual calculations. The variables of respiration of this subject monitored by the laboratory or clinical test system and results of said analysis would be compared. This type of calibration was crude at best since the biological subject could not produce uniform, controlled, respiration and metabolic rate.
A respiratory simulator has also been previously developed, by others, to simulate inspiration and expiration using a double piston pump (A Piston Pump for Respiration Simulation; U. Boutellier, U. Gomez, and G. Mader; J. Appl. Physiology, 50(3): 663-664, 1981). The Boutellier et al double piston simulator utilizes one cylinder as the inspiratory chamber, "inhaling" atmospheric air, while the other cylinder acted as the expiratory chamber, "exhaling" a pre-mixed O.sub.2 --CO.sub.2 --N.sub.2 gas mixture. While this system provides on-line analysis of known volumes and compositions of gases, it requires the use of an expensive pre-mixed gas for the calibration. Because the pre-mixed gas mixture was solely used as the expiratory gas, only one composition of expired gas could be used for calibration purposes. To vary compositions, different tanks of pre-mixed O.sub.2 --CO.sub.2 --N.sub.2 gases of exact known compositions would be required, and this approach to on-line calibration thus becomes very expensive.
In general, the invention relates to a respiratory simulator that replaces biological subjects to test the accuracy and calibrate the performance of respiratory analysis equipment. The simulation of respiration is accomplished by a single cylinder piston pump that inhales atmospheric air and mixes it with a calibration gas composed of CO.sub.2 and N.sub.2. The pump then "exhales" the mixture of gases at known concentrations and known frequencies and volumes and compares the results given by the analytical equipment. The present invention thus allows simplified on-line automatic calibration of respiratory and metabolic analysis equipment over a wide range of exhalation products.